This invention relates to the field of detecting unauthorized "bugs" on telephone lines. More particularly, this invention relates to apparatus and method for remotely determining the presence of an unauthorized surveillance device installed on a communications line between user terminal equipment and a central communications exchange station. Those having skill in the communications field will recognize that the teachings of this application may be adapted to any system for measuring desired parametric values in a communications loops. Such measurements may include the capacity of a line to handle data, the capability of a line to perform multi-channel communications, the ability of a communications loop to carry digital and/or analog signals, etc.
As sophistication in electronics increases, security threats to land-line communications multiply. Many devices have been designed to protect land-line communications security, and an entire industry has been built around such communications security. Most known devices are concerned with detecting surveillance devices attached to a telephone, or surveillance devices installed in a central telephone exchange station.
For example, U.S. Pat. No. 4,218,592 to Steinbergs et al discloses a communications security device which detects whether a telephone has been wired as a hidden listening device. This apparatus is meant to detect any devices which may convert a hung-up telephone into an active listening device.
Also known are "scrambler" devices which may be connected to a telephone and scramble the voice of a user in accordance with a given code. A receiver having a "descrambler" device provided with the same code may receive the scrambled signal and convert it to normal voice communications.
While much attention has been focused on providing security at the transmission and the reception ends of a telephone communication, little consideration has been given to protecting the telephone line between the transmitting and receiving devices. Yet, a well designed security system must ensure the security integrity of the telephone line as well as the transmitter and receiver.
One solution for ensuring the security of the telephone line is to provide "dedicated" lines which are ostensibly not subject to being "bugged". However, such "dedicated" telephone lines are very expensive to install and maintain, and must be continually guarded.
Also known are devices for checking fault conditions in a telephone line between a central telephone exchange station and a point in the telephone line between the exchange station and the user telephone. For example, U.S. Pat. No. 4,415,779 to Bowman discloses a method and apparatus for testing a telephone line to locate a fault with reference to a regulator box attached on the outside of the house of the telephone user. Bowman is concerned with determining whether a fault in the telephone line is located between the regulator box and the telephone company's switching station. Bowman teaches that one method for checking telephone line faults is to mechanically disconnect the telephone line at the regulator box and reconnect it to a termination network of known impedance. The known impedance is then measured in response to a test signal sent from a central office. If the measurement agrees with the known impedance, no fault lies between the regulator box and the central office.
Bowman provides a maintenance termination unit which is permanently installed next to the regulator box on the outside of the subscriber's home. During normal usage, the maintenance termination unit is transparent to the telephone user. In the test mode, a high voltage, continuous, unidirectional current is applied from the central office. In response to the signal, the maintenance termination unit determines whether there is a fault in the telephone line between the central office and the regulator box. However, Bowman is concerned with fault location and not with "bug" detection. Thus, while Bowman may be capable of detecting faults, he is not capable of determining the presence of sophisticated electronics eavesdropping equipment. Furthermore, Bowman is incapable of detecting a fault in the telephone line between the regulator box and the user's telephone inside his home. It is known that many eavesdropping devices are installed between the regulator box and the user's telephone. In addition, Bowman requires a maintenance termination unit to be located on the outside of the subscriber's home, next to the regulator box. In this location, the maintenance termination unit is highly susceptible to being tampered with, damaged, or otherwise circumvented.
Another example of a line testing device is disclosed in U.S. Pat. No. 4,413,163 Basini. Basini discloses a portable line tester for a telephone system. The portable line tester is carried by an operator to a location from which the test is to be conducted. For example, the line tester is taken to a location where a line fault is suspected. The portable line tester is attached to the telephone lines and then "calls" a generator installed at the central switching station. The generator responds by sending a pilot tone to the portable line tester. The portable line tester then measures the received pilot tone to determine the attenuation of the signal between the central switching station and the portable line tester unit. As in Bowman, Basini is incapable of determining the presence of a "bug" installed on the telephone line anywhere from the central switching station up to the user's telephone. Furthermore, Basini requires an operator to be dispatched to the location at which the line test is to be conducted.
Other known line test systems for detecting the presence of a line fault in telephone lines are disclosed in U.S. Pat. No. 4,388,501 to Ahuja, U.S. Pat. No. 4,446,340 to Fryer, U.S. Pat. No. 4,350,849 to Ahuja, and U.S. Pat. No. 4,524,321 Jablway et al. Like the above-discussed patents, these systems describe the use of scanners, central office and/or subscriber located devices, and other equipment which establish the existence of line faults, opens, shorts, and reversals along a telephone line. None of these patents addresses the detection of unauthorized surveillance devices installed on the telephone line.
Thus, there is an apparent need for a system which detects the presence of a surveillance device on a telephone line from a location remote from the "bug". Such a system should be easily installed, easy to maintain, and should not require an operator to be dispatched to conduct the "bug" checks.